[SGVLUG] Off-topic - ac vs. dc

[David Lawyer]

On Sat, Feb 24, 2007 at 12:38:18PM -0800, Dustin Laurence wrote:
> In addition to the issues already mentioned, AC allows the use of very
> simple brushless motors (there are brushless DC motors but IIRC they are
> much more complex--this was probably more of an issue back when we
> invested in the power grid than now, but paid-for infrastructure counts
> for a lot).  It's nice not to have to parts that wear out periodically,
> and IIRC brushes actually produce a great deal of electrical noise.

You're right re infrastructure, but if there isn't any substantial
infrastructure, like electric railroads, then DC would be better.  It
turns out that almost all of portable power tools today have brushes
and are really DC (but they run on AC although inefficiently).  That's
because at fixed frequency, AC motors turn at fixed speed while a DC
motor can easily vary it's speed.  For DC, by putting the field in
series with the rotating armature, they get high torque at low speed.

A DC system would operate a lot of AC motors and use inverters to
change DC to AC.  A big advantage would be the variable speed of AC
motors obtained by varying the frequency.  Building ventilation could
run slower when fewer people are using the buildings, etc.  Today
fixed speed AC motors are often used in applications where variable
speed motors would be better if it were not for the cost.  An inverter
starting with DC is likely to be cheaper since the first step of
changing the frequency of AC is to first rectify it to DC (or to
implicity do this).
[snip]
			David Lawyer